Torque limiting devices are commonly used to insulate the driven side of a transmission assembly from excessive intermittent surges of torque and/or continuous excessive torque applied by a driving, input side.
Exemplary of such a structure is that shown in U.S. Pat. No. 3,640,092, to Neal et al. In Neal et al, a fixed, cylindrical, outer race retains internally a cam constituting the input member and an associated roller cage constituting the output member and operatively engaged with a plurality of rollers. The input and output members are connected by a torsion spring or other suitable member capable of deflection when subjected to a predetermined input torque. The application of excessive torque through either the input or output member causes relative rotational movement between the cam and cage, thereby wedging the rollers to stall the input torque.
The structure in Neal et al has several drawbacks. The Neal et al structure is suitable for the control of but a single output. Consequently, where a plurality of torque responsive elements are driven from a single drive source, multiple torque limiting devices would have to be employed.
An alternative torque limiting arrangement is disclosed in U.S. Pat. No. 3,497,044, to Kalns. In Kalns, a pair of brake shoes are provided within and conformed substantially to a cylindrical drum surface formed interiorally of a housing. The brake shoes are maintained disengaged from the drum surface by an arrangement of springs which draws the paired brake shoes, each towards the other. Shoe actuating control pins are provided and maintain the shoes separated from one another against the force of the springs, with each pin projecting axially outwardly of the shoes and terminating in a rounded head which engages the input member. Under a predetermined excessive input torque, the pins tilt reactively and exert a spreading force on the shoes, thereby biasing the shoes into engagement with the drum surface, to damp the torque.
The structure in Kalns, as in Neal et al, is restricted to limiting the torque of a single output. Further, a complicated structure is required to retain the brake shoes in a retracted position as well as to cause engagement of the brake shoes with the drum surface at the predetermined torque. Still further, reliance on the pin tilting to engage the shoes and drum and single surface engagement between each brake shoe and drum results in non-positive and unpredictable torque control.
This invention is specifically directed to overcoming the problems enumerated above.